I'm a central office tech for AT&T, formerly worked in the business office, and I think we'd screw it up trying to do as you suggest. You might try turning up a second POTS line on your second pair, then later calling to install your new U-Verse in association with your new POTS. This wouldn't have to affect your existing DSL. In other words, make sure you have a second POTS in service, or they'll mess up your existing DSL to give you U-Verse.
